This is a Phase II, comparative study of seroconversion of single-dose and two-dose measles vaccination in HIV-infected and HIV-uninfected children. Despite postexposure prophylaxis with IgG or IVIG, HIV-infected children still contract measles, and the associated mortality is high. As vaccination of children at 15 months of age leaves a small percentage susceptible and unprotected, steps to enhance seroconversion and to prolong the duration of the antibody response are vital. To enhance seroconversion, this study will investigate the impact of administering the measles vaccine early in the first year of life. To prolong the duration of antibody, the study proposes to administer vaccine in two consecutive doses.